Upvote/Downvote System - php

Question

I have the following PHP script:
<?php
$vote_type = $_GET['type'];
$book = $_GET['book'];
$id = $_GET['id'];
include 'pagehead.php';
$tracker_table = $book.'VoteTrack';
$username = $_SESSION['username'];
session_start();
if ($_SESSION['username'] == null) {
echo 'You must be logged in to vote';
echo '<br>';
echo '<a href="lesson.php?book='.$book.'&id='.$id.'">';
echo 'Return to lesson';
echo '</a>';
die();
}
include 'mysqlserver.php';
$con = mysql_connect($mysql_host, $mysql_username, $mysql_password);
if (!$con){
die ('Failed to connect to the database');
}
mysql_select_db("a6595899_s", $con);
$data_query = "SELECT * FROM $book WHERE id=$id";
$lesson_data = mysql_query($data_query);
$lesson_array = mysql_fetch_assoc($lesson_data);
$vote_cop_query = "SELECT * FROM $tracker_table WHERE user='$username' AND id=$id";
$vote_cop_data = mysql_query($vote_cop_query);
$vote_cop = mysql_fetch_assoc($vote_cop_data);
if (mysql_num_rows($vote_cop_data) != 0 && $vote_type == 'up' && $vote_cop['has'] == 1) {
echo 'You have already upvoted this lesson.';
echo '<br>';
echo '<a href="lesson.php?book='.$book.'&id='.$id.'">';
echo 'Return to lesson';
echo '</a>';
die();
} elseif (mysql_num_rows($vote_cop_data) != 0 && $vote_type == 'down' && $vote_cop['has'] == 2) {
echo 'You have already downvoted this lesson.';
echo '<br>';
echo '<a href="lesson.php?book='.$book.'&id='.$id.'">';
echo 'Return to lesson';
echo '</a>';
die();
}
$vote_count = $lesson_array['votes'];
if ($vote_type == 'up') {
$vote_count++;
$has_type = 1;
} elseif ($vote_type == 'down') {
$vote_count--;
$has_type = 2;
} else {
die('Vote type not specified.');
}
$new_or = mysql_num_rows($vote_cop_data);
if ($new_or == 0) {
$track_query = "INSERT INTO $tracker_table (user, id, has)
VALUES ('$username', $id, $has_type)";
} else {
$track_query = "UPDATE $tracker_table SET has=$has_type WHERE user='$username' AND id=$id";
}
mysql_query($track_query);
//actually cast vote..
$update_query = "UPDATE $book SET votes=$vote_count WHERE id=$id";
mysql_query($update_query);
echo 'Your vote has been submitted!';
echo '<br>';
echo '<a href="lesson.php?book='.$book.'&id='.$id.'">';
echo 'Return to lesson';
echo'</a>';
?>
It's a very simple vote-up/and vote-down system. Unfortunately, it breaks down in certain scenarios. Let's say I'm reading a lesson that I think is good, so I vote it up. Later, I realize that the lesson is actually awful, so I downvote it. After I upvoted it the first time, the lesson had one point. After I downvote it, it has 0 again. Logic dictates that I should be able to downvote the lesson again, giving it -1 points. My code will not allow this, as my script simply says that the same action isn't allowed 2 times in a row.
What math do I use to fix this?

Answer 1

The problem is here where you're updating a user's activity after having downvoted their own upvote.
$track_query = "UPDATE $tracker_table SET has=$has_type WHERE user='$username' AND id=$id";
What you -should- be doing, is removing the record from the table instead of updating it, then go on to modify the score as you already are. That way, the next vote you do will be the 'first' vote you've done.
Alternatively, you could have a third vote_cop type called 'nullify' or 'revoke' or something then modify the vote cop accordingly.
See my suggestion below:
$hasVotedBefore = mysql_num_rows($vote_cop_data) != 0;
if ($hasVotedBefore) {
switch ($vote_cop_data['has']) {
case 0:
$vote_cop_type = 'revoked'; // Not really neccessary to do this, but just here for show.
break;
case 1:
$vote_cop_type = 'up';
break;
case 2:
$vote_cop_type = 'down';
break;
default:
break;
if ($vote_type == $vote_cop_type) { // We're here because we voted before and our new vote is the same as the old one.
if ($vote_type == 'up') {
echo 'You have already upvoted this lesson.';
echo '<br>';
echo '<a href="lesson.php?book='.$book.'&id='.$id.'">';
echo 'Return to lesson';
echo '</a>';
die();
} elseif ($vote_type == 'down') {
echo 'You have already downvoted this lesson.';
echo '<br>';
echo '<a href="lesson.php?book='.$book.'&id='.$id.'">';
echo 'Return to lesson';
echo '</a>';
die();
}
} else { // Were here because we've voted before, and our new vote is the opposite of the old vote.
// Update vote_cop row in the database so the 'has' column is 0 (value of a revoked vote)
// This way, for future votes, we know the user has voted before, but revoked their vote.
$track_query = "UPDATE $tracker_table SET has=0 WHERE user='$username' AND id=$id";
mysql_query($track_query);
}
} else { // We're here because we never voted before.
$track_query = "INSERT INTO $tracker_table (user, id, has) VALUES ('$username', $id, $vote_type)";
mysql_query($track_query);
}
// TODO: actually cast vote..